This invention relates to a band brake mechanism for an automatic transmission of an automobile, wherein brake bands are disposed around a ring gear of a planetary gear mechanism and around an outer housing of a wet multiplate clutch for loading them with braking force.
A band brake mechanism for an automatic transmission should be constructed in a lighter, more compact and more highly efficient structure than those for washing machines and other machines. For a highly efficient structure, a brake band hoop should be constructed so that it can be pressed to a brake drum with a constant pressure along its full length.
However if thin spring steel is used as the band hoop, and a thin friction lining is fixed to a band hoop for a compact, light and highly efficient structure, the following disadvantages may result. Namely, referring to FIG. 1, a band hoop 1' in free condition before setting was formed so that it might keep a constant space away from the full peripheral surface of a drum 6. However when the band hoop 1' is set as shown in FIG. 1, and a distance between both ends 2 and 3 of the hoop 1' is reduced, the band hoop 1' is deformed into an egg-like shape, and curves most sharply at a middle portion 4 which is most distant from the ends 2 and 3. Because of this deformation, when the load by piston 5 on the end 3 is released, points A and A' slightly touch the drum 6 and drag the drum 6. This results in the disadvantages that a friction lining fixed to the band hoop 1' is partially worn, and that fuel consumption per hour increases.
Accordingly, it is an object of the invention to provide an improved band brake mechanism, wherein the band hoop does not touch the brake drum at any point when the band hoop is not intentionally held against the drum.
Other and further objects, features and advantages of the invention will appear more fully from the following description of the preferred embodiment of the invention.